Chemical Engineering May 2022 - 42

different temperature
applications.
Glass
membranes
are also designed
for some specific
chemical interactions,
such as
sterilization
applications
or test solutions
containing
hydrofluoric
acid.
FIGURE 3. Ion exchange takes place at the glass membrane of a pH sensor
change across the membrane.
Because ion activity changes with
temperature, the characteristics of
the glass must be suitable for the
measuring conditions. Composition
of the glass membrane is a highly
confidential field of research for pH
sensor manufacturers. Different
glass membranes should be used for
Digital versus
analog sensors
Analog systems.
Analog pH measurement
installations
consist of a
sensor, a transmitter
and a cable connecting the two
together. The entire system works
together to measure the pH, which is
transmitted to a control system using
a 4-20-mA signal or digital communication
protocols. This means that
the sensor is not a stand-alone device,
but is dependent on the wiring
and the transmitter to form a measurement
loop.
A significant
disadvantage of
this approach is
that a change to
a
single component
affects
the pH measurement.
For
example, if the
wire between
the
probe and
transmitter is
changed, the
readings will
change, and the
entire system
must be recalibrated.
Digital
systems.
Digital pH
measurement
systems contain
all the components
for measurement
on
the sensor itself.
The cable from
the sensor to
the transmitter
supplies power
and transmits
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information digitally. Therefore, it
does not affect the measurement.
Changing the cables from the sensor
to the transmitter will not affect the
pH measurement, which eliminates
the need for recalibration. When replacing
a digital probe in the field,
it can also be immediately commissioned,
as long as it has been calibrated
in a laboratory setting. Digital
sensors are insensitive to moisture in
the connections like analog sensors
are, because these connections are
not part of the measurement loop.
The hazards of field work. The
chemical industry is often a hazardous
environment, which uses products
and materials that can cause
serious injuries to operators or
maintenance technicians. It is vital
to keep personnel out of the field as
much as possible to minimize the
risk of accidents. Digital pH probes
help achieve this because they can
be replaced without field calibration.
In contrast, analog systems must
be recalibrated in the field after any
component is changed, which prolongs
the time a person must spend
in the field working on the equipment.
This leads to a higher safety
risk and to increased maintenance
costs due to the time that it takes to
complete the task.
The benefits of information storage.
Digital sensors also have the
benefit of internal data storage,
which captures critical information
about the process and the condition
of the sensor.
Calibration data can be stored in
the head of a digital sensor. This enables
the device to be calibrated in
the laboratory before field installation.
The device will read accurately
without the need for field calibration
checks. Digital devices store their
original reference calibration along
with the records of previous calibrations.
This allows maintenance technicians
to optimize the calibration
frequency by monitoring the drift in
the readings over time.
Digital sensors also store critical
operating information. For example,
they record extreme temperature or
pH events that could reduce the life
span of the sensor or its current accuracy.
This information is vital for
predictive maintenance and for as
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